Datagrid ScrollViewer.ScrollChanged Event in CustomControl using WPF

As I realized, you have a custom WPF control that has a ScrollViewer in its ControlTemplate and, you want to add an event-handler to the ScrollChanged event of the ScrollViewer.

Since the RoutedEvent is bubbled from the control to the window via tha control's parents, you can catch the event in each parent of the path. So, if you want to handle this event in the window that holds your ComboBoxDatagrid control, you can catch the ScrollViewer.ScrollChanged event, in the ComboBoxDatagrid control like the following:

<myControls:ComboBoxDatagrid x:Name="cbdg1"
                                    ScrollViewer.ScrollChanged="cbdg1_ScrollChanged" />

and, in the event-handler you can use the vertical-offset like the following:

private void cbdg1_ScrollChanged(object sender, ScrollChangedEventArgs e)
{
    double d = e.VerticalOffset;

    // Here you can use the vertical-offset's value.
}

If you want to add an event-handler to the ScrollViewer.ScrollChanged event, in the window's code-behind, you can use the AddHandler method like the following:

public MainWindow()
{
    InitializeComponent();

    cbdg1.AddHandler(ScrollViewer.ScrollChangedEvent, new ScrollChangedEventHandler(cbdg1_ScrollChanged)); 
}

If you want to add an event-handler in the control's code, you can set a name to the ScrollViewer in the control's template like the following:

<Style TargetType="{x:Type myControls:ComboBoxDatagrid}">
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="{x:Type myControls:ComboBoxDatagrid}">
                <Grid>
                    <ScrollViewer x:Name="PART_ScrollViewer">
                        <!-- ... -->
                    </ScrollViewer>
                </Grid>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>

and use the GetTemplateChild method in order to get the ScrollViewer from the control's template like the following:

public override void OnApplyTemplate()
{
    base.OnApplyTemplate();

    ScrollViewer scrollViewer = GetTemplateChild("PART_ScrollViewer") as ScrollViewer;
    scrollViewer.ScrollChanged += OnScrollChanged;
}

void OnScrollChanged(object sender, ScrollChangedEventArgs e)
{
    double d = e.VerticalOffset;

    // Here you can use the vertical-offset's value.
}